Re: multiple parameters to an AGGREGATE function

On Fri, Feb 3, 2012 at 11:27 AM, Graham <graham(dot)stark(at)virtual-worlds(dot)biz> wrote:
> Hi,
>   first time poster here ...
>
> I'm trying to write a simple Aggregate function which returns the nth
> element in a collection - ultimately I want to find 95th, 90th percentiles
> and so on.
>
> It'd be called like:
>
> select nth_element( value, 95 ) from something group by ...
>
> I'm basing this on an example I found on the Wiki:
>
> http://wiki.postgresql.org/wiki/Aggregate_Mode
>
> So, I have:
>
> CREATE OR REPLACE FUNCTION _final_nth( anyarray, integer )
>  RETURNS anyelement AS
> $BODY$
>    SELECT a
>    FROM unnest( $1 ) a
>    ORDER BY a
>    offset $2
>    LIMIT 1;
> $BODY$
> LANGUAGE 'sql' IMMUTABLE;
>
> CREATE AGGREGATE nth_element( anyelement, integer ) (
>  SFUNC=array_append,
>  STYPE=anyarray,
>  FINALFUNC=_final_nth,
>  INITCOND='{}'
> );
>
> Where the 2nd parameter would be, say 5 for the 5th element an so on.
>
> The function declaration seems fine. But the CREATE AGGREGATE declaration
> fails with:
>
> ERROR:  function array_append(anyarray, anyelement, integer) does not exist
>
> so, I suppose it's decided to call array_append with all the parameters on
> the command line, rather than just the array in the 1st element. Is there
> any way to stop it doing this?

I'm too busy to check, but it's probably calling your function at the
end with the last element scanned. Just create it the way it wants
and log the arguments -- it should be usable.

merlin